The metastatic spread of tumor cells is the primary cause of cancer related mortality, indicating a need for therapeutic approaches capable of controlling or preventing metastasis (Gibbons et al. (2012) OncoImmunology 1(7):1061-1073; and Grivennikov et al. (2010) Cell 140:883-899). The presence of tumor-infiltrating effector and memory T cells is correlated with decreased metastatic spread, consistent with a role for T cells in preventing metastasis of primary tumors.
B7-H1 (also referred to as PD-L1) is a polypeptide expressed by a variety of tumor cells. It also is constitutively expressed by macrophages and dendritic cells, and its expression is up-regulated upon cell activation. PD-1 is expressed on the surface of activated T cells, B cells, and macrophages, and is a receptor for B7-H1. CD80 is found on activated B cells and monocytes, and provides a costimulatory signal necessary for T cell activation and survival; CD80 also binds B7-H1.